In recent years, an electronic technology called “spintronics” has been attracting lots of attention. While traditional electronics has utilized only “electric charge” that is one property of an electron, the spintronics also utilizes actively a “spin” which is another property of an electron in addition to the electric charge. In particular, a “spin-current”, that is, a flow of the spin angular momentum of an electron is an important characteristic. Since the spin-current has weak energy dissipation, there is a possibility of realizing highly efficient information propagation by utilizing the spin-current. Accordingly, it has become an important subject of research to generate, detect, and control the spin-current.
With respect to the generation of the spin-current, for example, a phenomenon has been known that the spin-current is generated if an electric current flows. This phenomenon is called the “spin Hall effect”. The reverse phenomenon has been also known that an electromotive force is generated if the spin-current flows. This is called the “inverse spin Hall effect”. By utilizing the inverse spin Hall effect, it is possible to detect the generation of the spin-current as that of an electromotive force or an electric current. The spin Hall effect and the inverse spin Hall effect both appear prominently in substances with large spin orbit coupling, for example platinum (Pt), palladium (Pd), and the like.
Recent research has revealed that the spin Seebeck effect appears in magnetic materials. The spin Seebeck effect is a phenomenon that the spin-current is induced, if a temperature gradient is applied to the magnetic material, parallel to the direction of the temperature gradient (see Japanese Patent Application Laid-Open Publication No. 2009-130070 and Japanese Patent Application Laid-Open Publication No. 2011-249746, for example). That is to say, a thermal spin-current conversion of converting heat into the spin-current is made to arise by the spin Seebeck effect. It is described in Japanese Patent Application Laid-Open Publication No. 2009-130070 that the spin Seebeck effect appears in a NiFe film, which is a ferromagnetic material. It is described in Japanese Patent Application Laid-Open Publication No. 2011-249746 that the spin Seebeck effect appears in the interface between a metallic film and a magnetic insulating material such as yttrium iron garnet (Y3Fe5O12:YIG).
Here, the spin-current induced by the temperature gradient can be converted into an electric field (an electric current or an electric voltage) by using the inverse spin Hall effect mentioned above. That is to say, it becomes possible to realize “a spin thermoelectric conversion” of converting the temperature gradient into electricity through the spin by utilizing both the spin Seebeck effect and the inverse spin Hall effect.
Next, the operation of the thermoelectric conversion element which uses such spin thermoelectric conversion technology will be described (see Japanese Patent Application Laid-Open Publication No. 2009-130070 and Japanese Patent Application Laid-Open Publication No. 2011-249746, for example). FIG. 9 is a schematic, perspective view to illustrate the operation of the thermoelectric conversion in a related spin-current thermoelectric conversion element. The related spin-current thermoelectric conversion element is configured to be a longitudinal type.
The related longitudinal spin-current thermoelectric conversion element is composed of a magnetic body and an electromotive body which are connected to each other. The magnetic body is magnetized in a negative x direction in FIG. 9. If the temperature gradient is applied in a negative z direction, a thermal spin-current flows in a positive z direction, that is, it flows from a high temperature region to a low temperature region. The thermal spin-current makes a pure spin-current arise in the electromotive body through a process called a spin injection near to the interface between the magnetic body and the electromotive body. Here, the spin injection is a phenomenon that the spin in the magnetic body, processing around the magnetization direction near to the interface between the magnetic body and the electromotive body, interacts with a conduction electron without spin in the electromotive body, and transfers or receives a spin angular momentum.
By the spin injection, the conduction electron with spin moves to the neighborhood of a spin injection interface in the electromotive body, and the pure spin-current is generated. In the pure spin-current, the same amounts of the conduction electron with up-spin and that with down-spin flow in opposite directions with each other. As a result, although there is no charge transfer, only spin angular momentum flows because the sign of the spin differs from each other.
In the following description of the present specification, it is referred to as being magnetically coupled to be in a state in which the spin injection phenomenon can arise. The spin injection phenomenon can arise not only in a case where the magnetic body is in immediate contact with the electromotive body but also in a case where they are so close to each other that the spin angular momentum can be transferred even if they are not in immediate contact with each other. That is to say, even though there is a void between the magnetic body and the electromotive body or an intermediate layer is inserted between them, there is a magnetic coupling if the spin injection phenomenon can arise.
If the electromotive body is made of a material with large spin orbit coupling, the transfer of the conduction electron arises by the inverse spin Hall effect. This conduction electron is transferred in a direction perpendicular to each of the spin-current direction and the magnetization direction. As a result, an electric current is generated which flows either in a positive y direction or in a negative y direction depending on the properties of the materials of the electromotive body.
In such a spin-current thermoelectric conversion element, the magnitude of electromotive force to be obtained can be calculated by multiplying the magnitude of the spin-current arising in the magnetic body by an injection efficiency of the spin-current at the interface between the magnetic body and the electromotive body, and an efficiency of conversion into the electromotive force by means of the inverse spin Hall effect in the electromotive body. Therefore, in order to improve the basic performance of the related spin-current thermoelectric conversion element, it is necessary to increase at the same time three indexes of the magnitude of the spin-current itself, the spin-current injection efficiency, and a spin-current to electric current conversion efficiency of the electromotive body.
The related technologies are described in Japanese Patent Application Laid-Open Publication No. 2005-285867 and Japanese Patent Application Laid-Open Publication No. H01-140701.